Method for operating an exhaust-gas purification system based on a catalytic absorption system

ABSTRACT

In a method for operating an exhaust-gas purification system, in which CO and NO are simultaneously oxidized in accordance with the SCONOx principle by means of a first catalyst, and the NO 2  formed is absorbed on the surface of the catalyst, in which furthermore SO 2  is oxidized in accordance with the SCOSOx principle by means of a second catalyst arranged upstream of the first catalyst, and the SO 3  formed is absorbed on the surface of the catalyst, and in which the first and the second catalyst are regenerated in a regeneration cycle at regular time intervals by means of a regeneration gas containing hydrogen, a reduced requirement for hydrogen is achieved and the overall performance of the purification system is enhanced by virtue of the fact that the regeneration of the first and second catalysts is carried out at time intervals of different lengths, and that the time interval between two regeneration cycles of the second catalyst is significantly longer than the time interval between two regeneration cycles of the first catalyst.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the area of exhaust-gas purification technology. It relates to a method for operating an exhaust-gas purification system.

[0003] 2. Discussion of background

[0004] Methods for removing nitrogen oxides (NOx) and carbon monoxide (CO) from the exhaust gases of diesel engines, gas turbines etc. without the need to use ammonia have been in development since the middle of the 1990s and have become known under the brand names SCONOx and SCOSOx.

[0005] In these exhaust-gas purification methods, certain gas components are first of all oxidized and then absorbed by means of special catalysts coated with platinum and potassium carbonate (SCONOx) or with platinum and a layer that absorbs S03 (SCOSOx). In the SCONOx process, CO is oxidized to CO₂ and NO is oxidized to NO₂, and the NO₂ is then converted to potassium nitrite (KNO₂) and potassium nitrate (KNO₃) by the potassium carbonate (K₂CO₃) In the SCOSOx process, SO₂ is oxidized to SO₃, and the SO₃ is absorbed. The SCOSOx process that removes sulphur precedes the SCONOx process because the latter would rapidly become ineffective due to sulphur components in the exhaust gas (in this regard, see U.S. Pat. No. 5,953,911).

[0006] With both processes, regeneration cycles must be inserted at regular intervals, and during these regeneration cycles the catalysts are removed from the exhaust-gas purification process and regenerated again with the aid of a special regeneration gas. In the case of the SCONOx catalyst, the potassium nitrite and potassium nitrate formed are converted into the original potassium carbonate, water and pure nitrogen in the presence of hydrogen and carbon dioxide from the regeneration gas. In the case of the SCOSOx catalyst, the SO₃ is converted into SO₂ and water in the presence of hydrogen and is removed from the absorbing layer. A detailed description of these processes can be found for example in the article by L. Czarnecki et al., SCONOX—Ammonia Free NOx Removal Technology for Gas Turbines, Proc. 2000 Int. Joint Power Generation Conf., Miami Beach, Fla., Jul. 23-26, 2000.

[0007] A typical SCONOx system for the gas turbine of a combined-cycle power plant or the like has ten to fifteen individually removable catalyst compartments, 80% of which are in the oxidation/absorption cycle and 20% are in the regeneration cycle at any one time. In this context, a regeneration cycle typically takes from 3 to 8 minutes, and each catalyst compartment is thus in the oxidation/absorption cycle for 12 to 32 minutes.

[0008] Hitherto, both the SCOSOx and the SCONOX catalyst have been regenerated during each regeneration phase. Since, in SCOSOx regeneration, the catalyst material must first of all be reduced, i.e. freed of adsorbed oxygen, before the liberation of the absorbed sulphur dioxide can begin, regeneration of the SCOSOx takes a relatively long time. Typically, 70% of the regeneration time is used for SCOSOx. In order to make available sufficient hydrogen for the regeneration of the SCONOx catalyst in the remaining 30% of the regeneration time, a relatively high concentration of hydrogen must be made available in the regeneration gas. This is achieved by as high as possible an operating temperature of the steam reformer used to produce hydrogen from natural gas (methane). However, a high operating temperature of the steam reformer is associated with considerable operating costs.

SUMMARY OF THE INVENTION

[0009] Accordingly, one object of the invention is to provide a novel method for operating a SCONOx/SCOSOx system that is distinguished by a reduced hydrogen requirement, improves the overall performance of the purification system and reduces the risk of deactivation of the SCONOx catalyst due to incomplete SCOSOX regeneration.

[0010] The object is achieved by the totality of features in claim 1. At the heart of the invention is the fact that the regeneration periods of the two types of catalyst are optimized independently of one another by carrying out the regeneration of the catalyst responsible for reducing sulphur dioxide much less often than that of the catalyst responsible for reducing NO and CO. The following advantages are achieved by this novel method of operation:

[0011] a. The hydrogen requirement for regeneration is reduced;

[0012] b. the risk of deactivation of the SCONOx catalyst due to incomplete SCOSOx regeneration is reduced; and

[0013] c. the performance of the system is enhanced.

[0014] The time interval between two regeneration cycles of the same catalyst is preferably selected as a function of the SO₂ content of the exhaust gas to be purified, the time interval between two regeneration cycles of the second catalyst being, in particular, between 6 and 24 hours.

WAYS OF IMPLEMENTING THE INVENTION

[0015] The proposal is to dispense with the regeneration of the SCOSOX catalyst responsible for reducing sulphur dioxide for a large part of the operating time of the system and to employ the entire regeneration time of each regeneration cycle to regenerate the SCONOx catalyst responsible for reducing CO and NO. Regeneration of the SCOSOX catalyst is carried out just once every 2 to 24 hours, depending on the S02 content of the exhaust gas.

[0016] This is possible because the storage capacity of the SCOSOX catalyst is sufficient to absorb the S02 in the exhaust gas of a gas turbine fired with natural gas for more than 48 hours before regeneration is necessary.

[0017] Operation according to the invention with a reduced SCOSOX regeneration frequency offers the following advantages:

[0018] a. A lower hydrogen concentration can be accepted in the regeneration gas since the entire regeneration time can be used for the regeneration of the SCONOx catalyst and there is thus more time to make available the quantity of hydrogen required for regeneration.

[0019] b. Owing to the relatively slow progress of SCOSOx regeneration, the liberation of SO₂ is generally not complete on completion of SCOSOx regeneration. The SO₂ that is still present in the catalyst chamber as a result can contribute to deactivation of the SCONOx catalyst and thus impair the action of the catalyst. Less frequent regeneration of the SCOSOx catalyst drastically reduces the risk that the SCONOx catalyst will be deactivated.

[0020] The method proposed can be employed in all SCONOx systems, i.e. not only in gas-fired systems but also in systems operated with low-sulphur diesel oil.

[0021] Use of the method proposed does not require any changes in the design of systems. Even the control software does not have to be modified since special SCOSOx regeneration is already provided at predetermined intervals.

[0022] Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein. 

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. A method for operating an exhaust-gas purification system, in which CO and NO are simultaneously oxidized in accordance with the SCONOx principle by means of a first catalyst, and the NO₂ formed is absorbed on the surface of the catalyst, in which furthermore SO₂ is oxidized in accordance with the SCOSOx principle by means of a second catalyst arranged upstream of the first catalyst, and the SO₃ formed is absorbed on the surface of the catalyst, and in which the first and the second catalyst are regenerated in a regeneration cycle at regular time intervals by means of a regeneration gas containing hydrogen, characterized in that the regeneration of the first and second catalyst is carried out at time intervals of different lengths, and in that the time interval between two regeneration cycles of the second catalyst is significantly longer than the time interval between two regeneration cycles of the first catalyst.
 2. The method as claimed in claim 1, characterized in that the time interval between two regeneration cycles of the second catalyst is selected as a function of the SO₂ content of the exhaust gas to be purified.
 3. The method as claimed in either of claims 1 and 2, characterized in that the time interval between two regeneration cycles of the second catalyst is between 2 and 24 hours. 